Add spatial audio, spacecraft entity, and level intro sequence

Distance-based sound effects with stereo panning for explosions, impacts,
and pickups. Spacecraft entity with full state machine (fly-in, land, take
off, fly-out), engine/synth sound loops, thruster particles, and PNG
spritesheet. Moon level intro defers player spawn until ship lands.

Also untrack build/ objects that were committed by mistake.

assets/levels/moon01.lvl

@@ -12,6 +12,8 @@ PARALLAX_STYLE 4
 MUSIC assets/sounds/algardalgar.ogg
 PLAYER_UNARMED
 
+ENTITY spacecraft 1 14
+
 ENTITY asteroid 66 1
 ENTITY asteroid 70 1
 ENTITY asteroid 74 1